Subterranean oil and gas wells are typically perforated utilizing a perforating gun suspended in the well from either a wireline or a tubing string. The gun may perforate either the formation of interest (for an uncased well), the casing and the formation (for a cased well).
Radially projecting shaped charges having a powder explosive have long been utilized in perforating guns to perform the actual perforation of the well. Prior art perforating charges are disclosed, for example, in U.S. Pat. Nos. 3,025,794 and 4,643,097. Such shaped charges are typical "set off" by primer cord, which in turn is activated by a booster charge. The booster charge may be fired by various techniques, such as by the impact of a go-devil dropped through the tubing string on a downwardly facing charge in the gun body.
The performance of a shaped charge is typically measured by the diameter and/or size of the entry hole (perforation) and the depth of penetration (total tip penetration) formed by the shaped charge. The particular subterranean environment in which the gun is utilized determines the desired performance of the shaped charge. In a gravel packed oil well, for instance, the total tip penetration is not particularly significant, although the size of the entry hole through the casing is very significant.
Those skilled in the art have long recognized that the positioning and spacing of the shaped charges in the perforating gun drastically affect the hydrocarbon recovery from the well. In certain formations, for example, shaped charges are spaced at 3" axial intervals along the gun body, while for other perforations a gun is utilized with shaped charges spirally spaced at 1" axial intervals to provide more perforations per foot through the casing.
Although various components of the shaped charge could theoretically be adjusted to alter the performance of the shaped charge, such components in practice are rarely changed due to safety, reliability, and cost considerations. The type and composition of the explosives used in the shaped charge are limited to obtain reliable yet safe operation of the perforating guns. The length and diameter of the charge case and thus the volume of explosive within the shaped charge are generally standardized to minimize manufacturing costs for gun bodies designed for positioning in certain diameter holes, e.g., within a 75/8" diameter casing.
Prior art perforating guns with shaped charges thus provide limited flexibility for modifying the performance of the shaped charge. The disadvantages of the prior art are overcome by the present invention, and improved techniques are hereinafter disclosed for both increasing the efficiency of a shaped charge, and for easily and inexpensively modifying the performance of the shaped charge in a perforating gun.